Frequency entrainment is an interesting phenomena in nonlinear vibrations. It was discovered in the 17th century by Christian Huygens, who remarked that two slightly out of step pendulum-like clocks became synchronized after they were attached to the same thin wooden board. Similar phenomenon, injection locking, was observed in radio frequency (RF) circuits and laser systems.
Entrainment of micromechanical device oscillation has also been achieved in the RF range. Oscillation induced by a continuous wave laser can be entrained to a frequency near the resonant frequency of the micromechanical device by applying a small pilot signal at a frequency close to the resonant frequency, or by modifying the effective spring constant of the resonator by imposing a small RF component on the laser beam intensity. Such oscillations may be difficult to precisely control.
When the mechanical oscillator is synchronized to the pilot signal, the variations in the frequency and phase of the mechanical oscillations are locked to and controlled by the frequency of the pilot oscillator. Such an implementation offers the possibility of phase tuning or modulating the feedback of the mechanial resonance, however it suffers from the lack of a constant RF output frequency which is a requirement for phased array radar or phase modulated communication systems.